|Publication number||US7813651 B2|
|Application number||US 12/555,790|
|Publication date||Oct 12, 2010|
|Filing date||Sep 8, 2009|
|Priority date||Jan 12, 2004|
|Also published as||CA2549466A1, CA2549466C, EP1738500A2, EP1738500A4, EP1738500B1, US7587144, US20050175358, US20090324251, WO2005067690A2, WO2005067690A3|
|Publication number||12555790, 555790, US 7813651 B2, US 7813651B2, US-B2-7813651, US7813651 B2, US7813651B2|
|Inventors||Vladimir Ilchenko, Lutfollah Maleki|
|Original Assignee||Oewaves, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (91), Non-Patent Citations (39), Referenced by (17), Classifications (13), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a divisional application of U.S. patent application Ser. No. 11/034,232, filed Jan. 11, 2005, now U.S. Pat. No. 7,587,144, which claims the benefits of the following U.S. Provisional Patent Applications:
No. 60/535,953 entitled “OPTO-ELECTRONIC TUNABLE MICROWAVE FILTER” and filed on Jan. 12, 2004; and
No. 60/609,450 entitled “TUNABLE MICROWAVE PHOTONIC FILTER” and filed on Sep. 13, 2004.
The entire disclosures of the above two patent applications are incorporated herein by reference as part of the specification of this application.
This application relates to radio frequency (RF) and microwave devices and photonic devices.
Various applications require filtering of spectral components in signals by selecting one or more spectral components while rejecting other components. One example is bandpass filtering where a selected one or more spectral components within a spectral band are selected to transmit and spectral components outside the spectral band are rejected. A filter may be tunable, e.g., under a control of a tuning control signal, to change the frequency range of the filtered signal. As an example, a radio receiver has a tunable filter to select a desired radio station broadcast signal from many radio broadcast signals at different frequencies in the air. A television tuner is another example of such tunable filters. Many RF and microwave tunable filters are made of electronic RF and microwave circuit components.
This application describes, among others, RF and microwave filters and filtering techniques for processing RF and microwave signals by using (1) photonic or optical components and (2) RF and microwave components. In some implementations, a part of the processing is performed in the RF and microwave domain such as applying a microwave or RF input signal to an optical modulator to control optical modulation of light, and another part of the processing is performed in the optical domain such as optical filtering of the modulated light to select one or more desired microwave or RF spectral components as the filtered output. The frequency of a selected spectral component can be tuned by either tuning the frequency of the light that is modulated by the optical modulator or an optical filter that is used to optically filter modulated optical beam.
In one implementation, a device described here includes an input port to receive an input microwave or RF signal, a laser to produce a continuous-wave laser beam, a first optical path to receive a first portion of the laser beam, and a second optical path to receive a second portion of the laser beam. The second optical path includes an optical modulator to modulate the second portion in response to the input signal to produce a modulated optical beam that carries the input signal, and a tunable optical filter to filter the modulated optical beam to select at least one spectral component in the input signal while rejecting other spectral components and to output a filtered modulated optical beam that carries the at least one selected spectral component. The tunable optical filter includes at least two optical resonators that are optically coupled to produce a filter function of at least a second order. A tuning control unit is provided in the device in this implementation to tune at least one of the two optical resonators to change a frequency of the at least one selected spectral component. In addition, an optical detector is provided to combine the first portion from the first optical path and the filtered modulated optical beam from the second optical path and to produce a filtered output signal comprising the at least one selected spectral component.
The device may use two whispering gallery mode (WGM) resonators as the two optical resonators which are tunable via an electro-optic effect. The tunable optical filter may include a third electro-optic whispering gallery mode resonator optically coupled to one of the two tunable optical resonators and tuned by the tuning control unit to effectuate a third order filter function in the tunable optical filter.
Alternatively, the tunable optical filter in the device may be implemented with a first optical waveguide optically coupled to the first and second optical resonators and to receive the modulated optical beam from the optical modulator, and a second, separate optical waveguide optically coupled to the first and second optical resonators to output the filtered modulated optical beam to the optical detector. The first and second optical resonators are directly optically coupled to each other in addition to optical coupling with each other via optical coupling to the first and second waveguides.
As another alternative, the tunable optical filter in the device may include a first optical waveguide optically coupled to the first and second optical resonators and to receive the modulated optical beam from the optical modulator and to output the filtered modulated optical beam to the optical detector, and a second, separate optical waveguide optically coupled to the first and second optical resonators. The first and second optical resonators are directly optically coupled to each other in addition to optical coupling with each other via optical coupling to the first and second waveguides.
Furthermore, the two optical resonators in the tunable optical filter of the device may be first and second optical resonators, respectively, and the tunable optical filter may further include third and fourth optical resonators. The first optical resonator receives the modulated optical beam from the optical modulator and the fourth optical resonator outputs the filtered modulated optical beam to the optical detector. The first, second, third and fourth optical resonators are optically coupled to one another in the following manner: the first optical resonator is optically coupled to the second and third optical resonators; the second optical resonator is further optically coupled to the fourth optical resonator; the third optical resonator is further optically coupled to the fourth optical resonator; and the second and third optical resonators are not directly coupled to each other and are indirectly coupled via the first and fourth optical resonators.
Other implementations described in this application perform the frequency tuning in the optical domain by tuning the frequency of the optical beam. For example, a method for filtering a signal includes applying a microwave or RF signal to an optical modulator to control optical modulation of an optical beam and to produce a modulated optical beam that carries the signal, optically filtering the modulated optical beam to reject undesired signal spectral bands in the modulated optical beam to produce a filtered optical beam that carries at least one selected signal spectral band, tuning a frequency of the optical beam to select the frequency of the at least one selected signal spectral band, combining a portion of the optical beam that is not modulated by the optical modulator and the filtered optical beam into a combined beam, and using an optical detector to convert the combined beam into a filtered microwave or RF signal that carries the at least one selected signal spectral band.
A device that implements the tuning of the frequency of the optical beam may include, for example, an input port to receive an input microwave or RF signal, a tunable laser to produce a continuous-wave laser beam and to tune a laser frequency of the laser beam, a first optical path to receive a first portion of the laser beam, a second optical path to receive a second portion of the laser beam, and a tuning control unit to tune the laser frequency of the tunable laser. The second optical path includes an optical modulator to modulate the second portion in response to the input signal to produce a modulated optical beam that carries the input signal, and an optical filter to filter the modulated optical beam to select at least one spectral component in the input signal while rejecting other spectral components and to output a filtered modulated optical beam that carries the at least one selected spectral component. Accordingly, the tuning control unit operates to tune the laser and thus change a frequency of the at least one selected spectral component. This device further includes an optical detector to combine the first portion from the first optical path and the filtered modulated optical beam from the second optical path and to produce a filtered output signal comprising the at least one selected spectral component.
In yet another implementation, a microwave or RF signal is applied to an optical modulator to control optical modulation of an optical beam and to produce a modulated optical beam that carries the signal. At least two cascaded optical resonators are used to optically filter the modulated optical beam to reject undesired signal spectral bands in the modulated optical beam to produce a filtered optical beam that carries at least one selected signal spectral band. A frequency of one of the two cascaded optical resonators is tuned to select the frequency of the at least one selected signal spectral band. A portion of the optical beam that is not modulated by the optical modulator and the filtered optical beam are combined into a combined beam. An optical detector is used to convert the combined beam into a filtered microwave or RF signal that carries the at least one selected signal spectral band.
These and other implementations, features, and associated various advantages are described in greater detail in the drawings, the detailed description, and the claims.
Tunable filters and filtering techniques described in this application use an input port to receive a non-optical input signal to be filtered, e.g., a microwave or RF signal, and an output port to export a filtered or processed non-optical signal, e.g., a filtered microwave or RF signal. The input signal is converted into optical domain via optical modulation of a continuous-wave optical beam and the modulated optical beam is then optically filtered to select desired microwave or RF spectral components. An optical filter with a high quality factor can produce ultra narrow linewidth to optically select one or more desired microwave or RF spectral components carried in the modulated optical beam. Such optical filtering of microwave or RF spectral components avoids use of microwave or RF filters that tend to suffer a number of limitations imposed by the electronic microwave or RF circuit elements. The filtered optical signal and a portion of the same continuous-wave optical beam are combined and sent into an optical detector. The output of the optical detector is used as the filtered or processed non-optical signal. Like the signal filtering, the frequency tuning of the filtering in these implementations is also achieved optically, e.g., by either tuning the frequency of the optical beam that is modulated by the optical modulator or an optical filter that is used to filter modulated optical beam.
In this specific implementation, the optical filtering and tuning of the output signal 102 are performed in the lower second optical path. The input RF or microwave signal 101 is first up-converted into the optical domain using a broadband modulator. The signal filtering is done in optical domain using a tunable high-Q optical filter. The signal tuning is also done in the optical domain by tuning the optical filter to select one or more spectral components. In the lower second optical path, an optical modulator 130, such as an electro-optic modulator, is used to modulate the second optical beam 112 in response to the input signal 101. This optical modulation produces a modulated optical beam 132 that carries the microwave or RF spectral components in the input signal 101. The operating bandwidth of the optical modulator 130 is designed to be sufficiently broad to cover the signal frequencies of the input signal 101. The microwave or RF spectral components in the input signal 101 appear as optical sidebands at different optical frequencies from the laser frequency of the laser 110. This process converts the microwave or RF spectral components into the optical domain. Therefore, signal filtering and frequency tuning can be performed optically.
Referring back to
The first optical beam 111 in the first optical path is not modulated and thus has only the optical carrier. When the first beam 111 and the filtered beam 145 are combined at the optical detector 160, the detection by the optical detector 160 presents the beat signal between the optical carrier and the filtered sideband in the detector 160. Therefore, the frequency of the output signal 102 from the detector 102 is the difference between the optical frequency of the filterted beam 145 and the first optical beam 111, i.e., the filtered RF sideband at the frequency of fRF. This converts the filtered signal down from the optical domain back to the RF and microwave domain. The optical filter 140 can be tuned to select any of the signal sidebands carried by the modulated optical beam 132. As such, the frequency of the RF signal 102 can be tuned.
The tunable optical filter 140 may be implemented in various configurations. For example, the tuning may be achieved by thermal control of the resonator whose index, dimension, or both change with temperature, mechanical control of the resonator by changing the dimension of the resonator, electrical control, or optical control. Electro-optic materials may be used to control and tune the resonance frequency of the WGM resonator by an external control signal. For example, a single lithium niobate microresonator that supports whispering gallery modes is a tunable optical filter based on the electro-optic effect of the lithium niobate material and can be used as the filter 140.
For example, a Z-cut LiNbO3 disk cavity with a diameter of d=4.8 mm and a thickness of 170 μm may be used as the resonator 210. The cavity perimeter edge may be prepared in the toroidal shape with a 100 μm radius of curvature. As an alternative to the strip electrodes shown in
Such a single-resonator filter has a Lorentzian lineshape in its spectral transmission and presents a less than ideal passband with a relatively slow roll-off from the center transmission peak. When the signal spectral bands in the input signal 101 are close to one another, the single-resonator filter may not be sufficient to separate neighboring bands. In various implementations, two or more such tunable microresonators may be optically cascaded together in series to create a multi-pole optical filter with a flatter passband and sharper spectral roll-offs. Light can be evanescently coupled between the closely-spaced (e.g., about 1 μm) or directly contacted microresonators.
The shape of the passband function for such a cascaded multi-resonator filter may be controlled by adjusting a number of device parameters. For example, the number of microresonators sets the order of the filter and directly determines how sharply the filter response rolls-off outside the passband. The quality factors of microresonators can determine the natural linewidth of the filter function. Tunable lithium niobate microresonators may be fabricated to produce varying bandwidths, such as narrow linewidths of about 10 MHz or less, or broad linewidths at tens of MHz. The physical gaps that separate the cascaded microresonators (and the coupling prisms at either end of the series from the first and last microresonators) can be adjusted to control the coupling strengths. The gaps may be fixed in certain implementations and adjustable for maximum flexibility in dynamically reconfiguring the filter function in other implementations. Different control voltages to different microresonators may be used to provide desired offsets of the different filter poles relative to a selected center of the filter passband to achieve a desired filter spectral profile. The tuning control unit 144 may include an embedded logic unit that dynamically adjusts the offsets of the filter poles. Accurate placements of the poles can minimize ripple in the final filter passband.
The design of multi-pole optical filters with microresonators may be analogous to design multi-pole RF filters to a certain extent but the design parameters are very different. For example, the equivalent RF Q factors of microresonators are much higher than many RF filters. The equivalent RF Q factor a Microresonator is the optical Q factor multiplied by a ration of the RF frequency over the optical frequency. Hence, at the optical wavelength of 1550 nm, the ratio is about 5×10−5 and an optical Q factor of 109 is equivalent to an RF Q factor of about 5×104.
A number of technical issues associated with implementation of multi-resonator filters are addressed below. The electro-optic effect in lithium niobate is evident in
Referring back to
The tunable optical filter 140 in
The specific examples described here are in optical domain and use optical waveguides and whispering gallery mode resonators. In particular, device designs with a parallel configuration of two interacting whispering-gallery-mode optical resonators are described to show a narrowband modal structure as a basis for a widely tunable delay line. The optical coupling can be optimized so that such devices produce an unusually narrow spectral feature with a much narrower bandwidth than the loaded bandwidth of each individual resonator.
This effect of the devices described here is analogous to the phenomenon of electromagnetically induced transparency (EIT) in resonantly absorbing quantum systems. The quantum-mechanical interference of spontaneous emissions from two close energy states coupled to a common ground state results in ultranarrow resonances in EIT. The devices and techniques described here produce similar narrow resonances based on classic cavity modes and the interference between direct and resonance-assisted indirect pathways for decays in two coupled resonators. This is the same Fano resonance for optical resonators that has been shown to result in sharp asymmetric line shapes in a narrow frequency range in periodic structures and waveguide-cavity systems.
The two resonators 510 and 520 may be spaced from each other so there is no direct optical coupling between the two resonators 510 and 520. Alternatively, the two resonators 510 and 520 may be directly coupled to each other to exchange optical energy without relying on optical coupling via the waveguides 501 and 502. Regardless whether there is a direct coupling between the two resonators 510 and 520, the two waveguides 501 and 502 provide an optical coupling mechanism between the resonators 510 and 520. In
In the design in
Therefore, the optical configuration of the tunable filter 500 provides an optical circulation and storage mechanism to circulate and retain light between the two resonators 510 and 520 and the segments of the waveguides 501 and 502 between the resonators 510 and 520. A portion of light circulating and stored in the device 500 is reflected back in the waveguide 502 as the reflected signal 523 and another portion of the light is transmitted through the two resonators 510 and 520 as the transmitted signal 522 in the waveguide 501.
The spatially overlapping and mixing of light from the two different resonators in
The transmission coefficient for the tunable device 500 in
where γ and γc are spectral linewidths caused by intrinsic cavity losses and coupling to the waveguides 101 and 102, respectively; frequencies ω1 and ω2 are resonance frequencies of modes of the resonators 510 and 520, respectively; the frequency ω is the carrier frequency of the input light; and ψ stands for the coupling phase that varies with the distance between the two resonators 510 and 520.
Consider a strong coupling regime γc>>|ω1−ω2|>>γ in the tunable device 500. Assuming the frequency tunings between the input light and the resonance frequencies of the two resonators 110 and 102, |ω−ω1| and |ω−ω2|, to be much less than the free spectral ranges of the two resonators 510 and 520 and let exp(iψ)=1, the power transmission of the tunable device 500 based on the above transmission coefficient shows two minima,
|TP|min 2≃γ2/4γc 2,
when the frequency of the input light is tuned to the resonance frequencies of the two resonators 510 and 520: ω=ω1 and ω=ω2 Notably, the power transmission of the device 510 also has a local maximum at the average frequency of the two resonance frequencies of the resonators 510 and 520, ω=ω0=(ω1+ω2)/2) /2. The local maximum is given by
This local maximum is the peak of a narrow transparency feature or transmission peak whose spectral position and linewidth can be tuned by tuning either one or both of the resonators 510 and 520.
That is, the frequency difference between the resonance frequencies of the two resonators 510 and 120 can be reduced to reduce the width Γ by tuning one or both of the resonators 510 and 520. The group time delay that is originated from the narrow transparency resonance in the transmitted light is approximately τg≈Γ−1:
Therefore the tunable device 500 can produce a large and tunable optical delay in transmitted light and operate as an efficient source of slow light. This tunable delay exceeds the minimum group delay available from a single resonator.
The origin of this subnatural structure in the transmission spectrum of the tunable device 500 with coupled resonators 510 and 520 lies in the interference of the optical decays in the resonators 510 and 520. In fact, in the overcoupled or strong regime considered here, the resonators 510 and 520 decay primarily into the waveguides 501 and 502 rather than the free space around the resonators 510 and 520. As such, there are several possible optical paths for photons transmitted through the resonators 510 and 520, and the photons may interfere because they are localized in the same spatial configurations determined by the waveguides 501 and 502. The optical transmission of the tunable device 500 is nearly canceled when the light is resonant with one of the resonant modes, ω1 or ω2, of resonators 510 and 520. However, the interference between the resonators 510 and 520 results in a narrow transmission resonance.
The tunable device 500 and other devices described here based on coupled optical resonators as optical delay lines and optical filters have several advantages over the atomic, slow light systems. For example, the resonator-based devices described here produce an optical delay that depends on the frequency difference (ω1−ω2) between the resonant frequencies of the two resonators. Since at least one of the resonators in the devices described here is a tunable resonator, this frequency difference can be tuned to tune the delay time. The tuning may be accomplished easily, for example, by use of resonators made from electro-optic materials such as certain crystals like lithium niobate crystals. The delay time corresponds to linewidth of the device. Hence, the linewidth can be changed or tuned by tuning one or more tunable resonators in the device. This tunable linewidth may be changed in a wide range based on the designs of the resonators, e.g., from hundreds of kilohertz to several gigahertz.
Another advantage of the current devices is that the frequency of the transparency peak is the average frequency of the two resonance frequencies of the two resonators 510 and 520, [(ω1+ω2)/2], and thus is arbitrary in the sense that it is tunable by changing either or both of the resonance frequencies ω1 and ω2. Notably, the frequency of the transmission peak is continuously tunable in a wide tuning range and thus can be tuned to any desired frequency within the tuning range. This tuning capability is desirable in many applications such as devices or modules that use optical filtering devices and optical switching devices.
In addition, the resonator-based devices described here can be designed to have much lower optical losses because WGM resonators may be designed and manufactured to have very high quality factors on the order from 106 to 109.
The tunable device 500 in
In the above examples, two adjacent optical resonators are not directly coupled to each other but are indirectly coupled via the waveguides 501 and 502. Alternatively, two adjacent optical resonators, such as 510 and 520, may be directly coupled to each other provide direct exchange of energy between the resonators 501 and 502 in addition to the optical coupling via the waveguides 501 and 502. As one example, the two resonators 510 and 520 may be sufficiently close to or in direct contact with each other to exchange optical energy via evanescent fields of the resonant modes of the two resonators. In addition, an optical coupling element may be used between the two resonators 510 and 520 to provide this direct optical coupling without relying on the waveguides 501 and 502. Examples of the optical coupling element include an optical grating, which may be inserted between the resonators or directly formed on the surface of at least one resonator, and a photonic band gap material inserted between the resonators. This direct optical coupling between two adjacent optical resonators in combination with the optical coupling via the waveguides provides unique spectral features in devices for high-order optical filtering.
Referring back to
One notable effect of the added direct coupling in device 700 is that a third-order filter function can be generated with the two resonators 710 and 720. This is in contrast to previous belief that a second-order filter function is generated by cascading two WGM resonators.
The device 700 may have one or more resonators that are tunable to tune the spectral response of the device 700. Similar to the device 700 in
The above specific examples of tunable RF or microwave filters based on optical filtering and tuning use optical tunability of the optical filter 140 in
Hence, the optical tuning may be achieved by tuning either one or both of the optical carrier frequency of the optical beam and the center frequency of the transmission passband of the optical filter. In some implementations, it is beneficial to use a tunable filter as shown in
Specific tunable RF and microwave filters with tunable lasers and fixed optical filters are described below as examples.
If the laser 910 is subsequently tuned to change the optical carrier 1020 to a different optical carrier, e.g., the optical carrier 2 at a lower frequency then the initial optical carrier 1, this tuning shifts frequencies of the modulation sidebands 1021 and 1022 to lower frequencies by the same amount. This change in the optical carrier frequency places a different part of the upper modulation sideband 1022 within the fixed passband 1030 of the optical filter 920 to select a signal band with a higher frequency f2 as the filtered output signal 102 from the optical detector 160.
This use of the tunable laser 910 for tuning the frequency of the filtered RF or microwave signal 102 can simplify the construction of the optical filter 920 because a fixed filter can be used as the filter 920 without the frequency tuning mechanism. Tunable multi-pole optical filters can be complex because changes in the multi-pole variants are to be synchronized during the tuning in order to maintain the desired multi-pole filter function. One or more resonators used in the fixed filter 920 may still be tunable filters to allow for tuning of individual resonators by the electro-optic or other effects to set the desired offsets of resonance frequencies so that a desired initial spectral profile of the filter passband can be achieved. Alternatively, UV-sensitive materials may also be used to form the resonators for the filter 920 so that UV trimming can be used to modify the refractive indices of the resonators and thus control the resonance frequencies of the resonators by exposing the resonators to UV light. After the initial filter profile is set, the optical filter 920 may be stabilized. The RF filter tuning is then achieved by tuning the laser frequency.
Agile frequency tuning in lasers, such as diode lasers and diode-based lasers, is well developed and can be implemented by different methods. For example, the driving current in distributed feedback (DFB) semiconductor lasers can be changed to tune the laser frequencies. Typical range of frequency tuning in some DFB lasers in the communication band 1550 nm is about 60-80 GHz, with an optical laser linewidth of about 1 MHz. Such tunable lasers are suitable for use in tunable RF or microwave filters with a tunable transmission passband of about 20 MHz and more.
Only a few implementations are disclosed. However, it is understood that variations and enhancements may be made.
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|U.S. Classification||398/183, 398/115, 398/187|
|Cooperative Classification||H04B10/505, G02F2203/055, H04B10/60, G02F2203/15, G02F2201/16, G02F1/011|
|European Classification||H04B10/60, H04B10/505, G02F1/01C|
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